Evaluation of enhanced high-resolution laser Doppler imaging in an in vitro tube model with the aim of assessing blood flow in separate microvessels

An enhanced high-resolution laser Doppler imaging (EHR-LDI) technique intended for visualization of separate microvessels was evaluated by use of in vitro flow models. Tn EHR-LDI, a laser beam focused to a half-power diameter less than 40 mu m successively scans the tissue under study in steps of 25 mu m. Spatial blood now variations within microvascular structures of 1.5 x 1.5 mm are rendered by 64 x 64 measurement sites. Individual plicrovessel diameters could be estimated and an average difference of 11 mu m compared to microscopic measurements was obtained. For the flow algorithm used, the LDI output signal was found to scale linearly with average velocity (0-3.5 mm/s) when a plastic tube of inner diameter 175 mu m was perfused with human blood (correlation coefficient 0.99). The LDI output signal was further found insensitive to hematocrit variations in the range 16-44%. Due to the limited laser light penetration in blood, a reduction in the LDI output signal was observed as the inner tube diameters were successively changed from 280 to 1400 mu m. (C) 1998 Academic Press.